1. Field of the Invention
This invention relates to a one shot circuit and more specifically to a one shot circuit having an extended output pulse length.
2. Description of the Prior Art
One shot circuits are well known in electronics. A one shot circuit ("one shot") delivers one output pulse for each input (trigger) pulse. The output pulse is a signal of a predetermined length (i.e., having a particular amplitude for a predetermined time) whereas the input pulse is typically a single upwards or downwards signal transition. These one shot circuits have wide application.
A typical prior art integrated circuit one shot circuit is shown in FIG. 1 and includes an input terminal 10 and input inverter 12. Three inverters 14, 16, 18 are connected serially to a first input terminal of a NOR gate 20. The combination of the delay provided by the three inverters 14, 16, 18 summed together is the duration of the one shot output pulse at terminal 24 via output inverter 22. Typically, this pulse is relatively brief, i.e. only 10 to 20 nanoseconds.
A one shot circuit of this type is not capable of delivering an output pulse of substantially longer length such as hundreds of nanoseconds or microseconds. This is because a one shot circuit delivering such an output pulse requires very large inverters 14, 16, 18, i.e. with very large powerful transistors, which is not economically feasible for large scale integrated circuit applications. Also, such a circuit would be unrealizable in a large scale integrated circuit because of noise. The noise is a result of the need to switch the inverters slowly, lengthening the time delay. The resulting very long rise and fall time render the circuit susceptible to noise.
Thus, it has not been known heretofore how to provide a one shot which is part of a large scale integrated circuit and has a long duration output pulse. The relatively long duration output pulse is desirable for many applications. A typical application of such, a one shot device in an integrated circuit chip is as an input circuit for receiving a signal from outside the chip at a chip pin and converting the input signal into a pulse of uniform length and amplitude for use by the logic circuitry in the integrated circuit. In this situation the input signal is often relatively noisy and hence contains glitches, i.e. is not a smoothly formed waveform.